This invention relates generally to receiver selection and more particularly to a system for selecting the highest quality signal from a plurality of received digital signals.
To implement a total area coverage or wide area coverage sytem, it is known in the at to provide a plurality of receiver sites to receive and relay the same information signal to a central receiving location. Generally, one or more of the plurality of received signals will have superior signal quality when compared to the other signals. In the mobile or portable communication environment, the signal having the highest signal quality may rapidly change as the mobile transceiver proximity to a particular receiver site changes. Accordingly, it is necessary to continually determine which of the incoming plurality of received signals has superior signal quality and select that signal to be received at the central receiving location.
In an analog modulation communication system, it is known to select the signal upon the basis of signal strength or received signal power. This simple criteria is generally sufficient in an analog system since the relative phases of the received signal are less critical to the selection criterion thatn in a digital modulation system.
However, to select a signal from a plurality of received digital signals, signal strength is generally not a sufficient criterion upon which to base the signal selection because of the additional parameter of the signal bit-error-rate (BER). A signal may have a higher signal power compared to other received signals and also have a higher BER thereby making that signal undesirable overall. Some selection systems have simply reconverted the digital signal into an analog signal and compared signal power as in the prior analog modulation communication systems.
In a secure communication system, decrypting the signal is undesirable since to decrypt the signal a receiver site requires the encryption key used to encrypt the digital signal. Remote decoding is contrary to the security of the system since there will be sections in the receiver where the voice message is no longer secure. Further, it is common in secure communication systems to periodically change the encryption key to prevent the compromise of system security. This is particularly disadvantageous in a wide area coverage system that employs several receivers in a diverse geographic ara. To change the encryption key in each receiver would result in wasted time and inefficient system use since the system would be inoperative until all receivers were programmed with the new encryption key.
Some digital selection systems have combined the plurality of received signals into a majority signal on the theory that if a majority of the received signals "thinks" the received bit should be a logical "1" the resultant majority signal BER will necessarily be superior to any of the individual signals. Of course, the majority signal may not always have a superior BER since the mobile or portable transceiver could be in close proximity to a particular remote receiver site, thus providing one very high quality signal within the plurality of received signals. Conversely, it is also possible to be in an area where all of the plurality of received signals are of poor quality and the resultant majority signal quality is better than any of the particular received signals.
Other selection systems have improved upon the majority signal technique by comparing each of the incoming signals to the majoriy signal. If a particular received signal is of extremely poor quality (i.e., has a high BER) that signal is excluded from contributing to the majority signal so that the majority signal BER will not be degraded by the contribution of that particular signal. However, this is generally not desirable in a mobile environment because the transceiver may rapidly relocate to be within close proximity of the received signal that has previously been excluded due to its prior poor BER. This results ina lower quality majority signal since the now "best" signal is excluded from contributing to the majority signal.
Accordingly, the need exists to have a simple, inexpensive but reliable signal selection method that overcomes the detriments of the prior art.